1. Field of the Invention
Mass Spectrometers often employ multipole ion guides including collision cells. Ion guides include a plurality of electrodes to which a variety of voltages are applied to contain or move ions radially and/or axially. The present invention relates specifically with apparatuses and methods for moving ions axially by auxiliary rods in multipole ion guides and collision cells.
2. Discussion of the Related Art
In tandem mass spectrometers such as triple stage quadrupole mass spectrometers, and also in other mass spectrometers, gas within the volumes defined by the RF rod sets in ion guides and collision cells improves the sensitivity and mass resolution by a process known as collisional focusing. In such a process, collisions between the gas and the ions cause the velocities of the ions to be reduced, causing the ions to become focused near the axis. However, the slowing of the ions also creates delays in ion transmission through the rod sets, and from one rod set to another. While the focusing is desirable, the slowing of the ions is also accompanied by other undesirable effects.
For example, when a rod set of an ion guide transmits ions from an atmospheric pressure ion source into a mass filter, the gas pressure in the ion guide may be relatively high (e.g. above 5 millitorr for collisional focusing) and collisions with the gas can slow the ions virtually to a stop. Therefore, there is a delay between ions entering the ion guide and the ions reaching the mass filter just downstream. This delay can cause problems in multiple ion monitoring, for example, where several ion intensities are monitored in sequence. If these multiple ions are monitored at a frequency which is faster than the ion transit time through the ion guide, then the fact that at least some of the ions are slowed to a stop has the negative impact of also causing the ions to have a sequence and a reduced rate at which the ions can be detected. The sequence and rate at which the associated data is processed and saved is also affected. In this case the signal from ions entering the ion guide may never reach a steady state. Thus, the measured ion intensity may be too low and may be a function of the measurement time.
Similarly, after product ions have been formed in a collision cell downstream of a first mass filter, for example, the ions may drain slowly out of the collision cell because of their very low velocity after many collisions. The ion clear out time (typically several tens of milliseconds) can cause tailing in the chromatogram and other spurious readings due to interference between adjacent channels when monitoring several parent/fragment pairs in rapid succession. To avoid this, a fairly substantial pause time is needed between measurements. The tailing also requires a similar pause. This required pause time between measurements reduces the productivity of the instrument.
In order to move ions axially through the multipoles forming ion guides and collision cells, it is known that the ions can be moved by segmentation of auxiliary rods and the application of voltages to the segments to create a voltage gradient along a length of the multipoles.
Background information for such a method is described in U.S. Pat. No. 5,847,386, entitled, “Spectrometer With Axial Field,” issued Dec. 8, 1998, to Thompson et al., including the following, “In a mass spectrometer, typically a quadrupole, one of the rod sets is constructed to create an axial field, e.g., a DC axial field, thereon. The axial field can be created by tapering the rods, or arranging the rods at angles with respect to each other, or segmenting the rods, or by providing resistively coated or segmented auxiliary rods, or by providing a set of conductive metal bands spaced along each rod with a resistive coating between the bands, or by forming each rod as a tube with a resistive exterior coating and a conductive inner coating, or by other appropriate methods.”
Background information on another segmented auxiliary rod structure is described in U.S. Pat. No. 5,576,540, entitled, “Mass Spectrometer With Radial Ejection,” to Jolliffe, issued Nov. 19, 1996, including the following, “Each rod 140 is divided into a number of axial segments 140-1 to 140-7, separated by insulators 141 . . . . The voltages on rods 140 create an axial DC field along the central longitudinal axis 142 of the rod set 132.”
Background information on other auxiliary electrode structures can also be found in U.S. Pat. No. 3,147,445 to Wuerker et al., U.S. Pat. No. 6,713,757 to Tanner et al., U.S. Pat. No. 6,909,089 to Londry et al and in U.S. Pat. No. 5,783,824, entitled “Ion Trapping Apparatus,” issued Jul. 21, 1998, to Baba et al.
The U.S. Pat. No. 7,067,802 to Kovtoun teaches an alternative way of forming an axial voltage gradient for moving ions through a multipole by applying a resistive path to an outer surface of the main electrodes of a multipole and applying a DC voltage to the resistive path.
The U.S. Pat. No. 7,084,398 to Loboda et al. teaches a method of selectively axially ejecting ions from a trap. The abstract explains that the method includes “ . . . separating the ions into a first group of ions and a second group of ions by providing an oscillating axial electric field within the rod set to counteract the static axial electric field . . . ”.